1. Field of the Invention
The present invention relates to techniques for a semiconductor integrated circuit and its driving method. The invention also relates to a light emitting device that has a semiconductor integrated circuit of the present invention in its driving circuit portion and a pixel portion, in particular, an active matrix light emitting device which has a semiconductor integrated circuit of the present invention as a signal line driving circuit in a driving circuit portion, which has a plurality of pixels arranged so as to form a matrix pattern, and which has a switching element and a light emitting element in each of the pixels.
2. Description of the Related Art
In recent years, development of light emitting devices using self-luminous light emitting elements has progressed. Making good use of advantages such as high quality image, thinness and lightweightness, such light emitting devices are widely used in display screens of mobile phones and personal computers. In particular, light emitting devices using light emitting elements are characteristic in that they have suitably fast response speed for animated displays, and low voltage and low power consumption driving. Thus, light emitting devices using light emitting elements are expected to be widely used for various purposes, including new-generation mobile telephones and personal digital assistants (PDAs) and are attracting attention as the next-generation displays.
An example of a light emitting element is an organic light emitting diode (OLED) with an anode and a cathode, and has a structure in which an organic compounded layer is sandwiched between the aforementioned anode and cathode. The organic compound layer generally has a laminate structure of which is represented by a laminate structure of “hole transport layer, light emitting layer, and electron transport layer”, proposed by Tang, Eastman Kodak Company.
In order to make a light emitting element emit light, the semiconductor device which drives the light emitting element is formed of polysilicon (polycrystalline silicon) which has a large ON current. The amount of current that flows into the light emitting element and the luminescence of the light emitting element are in direct proportion to each other, whereby the light emitting element emits light having luminescence in accordance with the amount of current which flows to the organic compound layer. Also, as the semiconductor device that drives the light emitting element, a polysilicon transistor formed of polysilicon is used.
However, when displaying a multi-gray scale image using a light emitting device with a light emitting element, a method of driving the device such as an analog gray scale method (analog driving method), or a digital gray scale method (digital driving method) can be given. The difference between the two lies in their methods of controlling the light emitting element in the state of light emission or non-light emission. The former analog gray scale method uses an analog method of controlling the current that flows into the light emitting element thereby obtaining gray scale. The latter digital gray scale method uses a method in which the light emitting element is driven in only two states, an ON state (almost 100% luminescence), and an OFF state (almost 0% luminescence).
Further, proposed is a current input method with which it is possible to classify the type of signal that is inputted into the light emitting device using the light emitting element as an example. In this current input method, it is supposed control of the amount of current that flows to the light emitting element is possible without being influenced by the TFT which drives the light emitting element.
The current input method is applicable to both the analog gray scale method and the digital gray scale method mentioned above. The current input method is a method where a video signal inputted into a pixel is a current and the luminescence of the light emitting element can be controlled by flowing current according to the inputted video signal (current) into the light emitting element.
Next, an example of a circuit construction of a pixel using a current input method and a driving method thereof in light emitting device will be explained with reference to FIG. 14. In FIG. 14, a pixel has a signal line 1401, first to third scanning lines 1402 to 1404, a power source line 1405, transistors 1406 to 1409, a capacitor element 1410, and light emitting element 1411. A current source circuit 1412 is provided to the signal line.
The transistor 1406 has a gate electrode connected to the first scanning line 1402. A first electrode of the transistor 1406 is connected to the signal line 1401 whereas its second electrode is connected to a first electrode of the transistor 1407, a first electrode of the transistor 1408, and a first electrode of the transistor 1409. The transistor 1407 has a gate electrode connected to the second scanning line 1403. A second electrode of the transistor 1407 is connected to a gate electrode of the transistor 1408. A second electrode of the transistor 1408 is connected to the current line 1405. The transistor 1409 has a gate electrode connected to the third scanning line 1404. A second electrode of the transistor 1409 is connected to one of electrodes of the light emitting element 1411. The capacitor element 1410 is connected between the gate electrode and second electrode of the transistor 1408 to hold the gate-source voltage of the transistor 1408. The current line 1405 and a cathode of the light emitting element 1411 receive given electric potentials to hold an electric potential difference with each other.
Operations from video signal writing to light emission will be described next. First, pulses are inputted to the first scanning line 1402 and the second scanning line 1403 to turn the transistors 1406 and 1407 ON. A signal current flowing in the signal line 1401 at this point is denoted by Idata and is supplied from the current source circuit 1412.
Right after the transistor 1406 is turned ON, no electric charges are held in the capacitor element 1410 yet and therefore the transistor 1408 remains OFF. In other words, a current caused by electric charges accumulated already in the capacitor element 1410 alone is flowing at this point.
Thereafter, electric charges are gradually accumulated in the capacitor element 1410 to cause a difference in electric potential between the electrodes. As the electric potential difference between the electrodes reaches a threshold Vth of the transistor 1408, the transistor 1408 is turned ON to generate a current flow. The current flowing into the capacitor element 1410 then is gradually reduced. However, the reduced current does not stop ongoing accumulation of electric charges in the capacitor element 1410.
Accumulation of electric charges in the capacitor element 1410 continues until the electric potential difference between its two electrodes, namely, the gate-source voltage of the transistor 1408, reaches a given voltage, which is a voltage (VGS) high enough to cause the current Idata to flow in the transistor 1408. When the accumulation of electric charges is finished, the current Idata continues to flow in the transistor 1408. A signal writing operation is conducted as above. Lastly, the first scanning line 1402 and the second scanning line 1403 stop being selected to turn the transistors 1406 and 1407 OFF.
A light emission operation follows next. A pulse is inputted to the third scanning line 1404 to turn the transistor 1409 ON. With the transistor 1408 turned ON by VGS which is written in the preceding operation and kept in the capacitor 1410, a current flows from the current source line 1405. This causes the light emitting element 1411 to emit light. If the transistor 1408 is set to operate in a saturation range at this point, a light emission current IEL flowing in the light emitting element 1411 does not deviate from Idata even when the source-drain voltage of the transistor 1408 is changed.
As described above, the current input method refers to a method in which a drain current whose current value is equal to or in proportion to the signal current value set by the current source circuit 1412 flows between the source and drain of the transistor 1408 and the light emitting element 1411 emits light with a luminance according to the drain current. By employing a current input method pixel as the one described in the above, influence of fluctuation in characteristic between transistors that constitute the pixel can be reduced and a desired current can be supplied to its light emitting element. Other current input method pixel circuits have been reported in U.S. Pat. No. 6,229,506 B1 and JP 2001-147659 A.
In a light emitting device employing the current input method, a signal current exactly reflecting a video signal has to be inputted to a pixel. However, when polysilicon transistors are used to build a driving circuit that inputs a signal current to a pixel (the circuit corresponds to the current source circuit 1412 in FIG. 14), characteristic fluctuation between the polysilicon transistors leads to fluctuation in signal current and unevenness in an image displayed. The characteristic fluctuation is caused by defects in crystal growth direction and grain boundaries, nonuniformity in thickness of the laminate, and insufficient accuracy in patterning a film. Because of large fluctuation between the polysilicon transistors, it is difficult to generate an accurate signal current and an image displayed will be full of streaks running vertically.
In other words, influence of characteristic fluctuation between transistors constituting a driving circuit that inputs a signal current to a pixel has to be reduced in a light emitting device employing the current input method. This means that influence of characteristic fluctuation has to be reduced both in transistors that constitute the driving circuit and in transistors that constitute a pixel.